(1) Field of the Invention
This invention relates to a process for manufacturing a double oriented electrical steel sheet including crystallized grains whose easy axis &lt;001&gt; of magnetization is oriented both in the longitudinal orientation and in the direction vertical thereto, together with the rolled surfaces exhibiting {100} planes (those crystallographic orientations can be represented as {100} &lt;001&gt; in the Miller indices).
(2) Description of the Related Art
Since the double oriented electrical steel sheet has excellent magnetic properties in the two different directions (e.g. B.sub.8 values both in the rolled direction and in the direction vertical thereto: 1.92 Tesla), because of its easy axis (&lt;001&gt; axis) in the rolled direction and in the direction vertical thereto, it can be more advantageously used for a magnetic core material of a specific apparatus, e.g. a large-scale rotating machine, where the magnetic flux flows in two different directions in comparison with a grain oriented electrical steel sheet which exhibits excellent magnetic properties in only one rolled direction. Non-oriented electrical steel sheets, whose easy axis is not greatly accumulated, have generally been used for a small stationary machine or installation. The use of a double oriented electrical steel sheet, therefore, makes it a possible to miniaturize the machine with an increased efficiency.
The double oriented electrical steel sheet, which has excellent magnetic properties as described above, has long been expected to be put into mass production, but the general use of such a type of sheet as an industrial product is still limited at present. Although various methods have been suggested, these are all only on a laboratory scale and have problems in terms of the industrial scale of the process.
As a prior art technique, a method wherein an initial steel sheet is annealed at a high temperature in an atmosphere containing a polar gas, e.g., hydrogen sulfide, to secondarily recrystallize out {100} &lt;001&gt; oriented grains with the aid of surface energy is described in Japanese Examined Patent Publication No.37-7110. Nevertheless, this method is inadequate for mass production, because it requires a very accurate control of the surface energy of the sheet. The other method is that wherein a steel sheet is cold-rolled in one direction and further cold-rolled in the direction vertical thereto, i.e. a "cross cold-rolling method", as described in Japanese Examined Patent Publication No. 35-2657 by Satoru Taguchi et. al. According to the cold-rolling method, a relatively higher magnetization property (B.sub.8 value) can be obtained, but the resulting product does not have a magnetization property which offsets the cost, and thus cannot replace the conventional grain oriented electrical steel sheet.
The magnetization property "B.sub.8 value" of the grain oriented electrical steel sheet has been significantly improved since the technique disclosed in Japanese Examined Patent Publication No. 40-15644 and were invented. The B.sub.8 value of equal to or more than 1.88 Tesla is standardized by JIS (Japanese Industrial Standard), and products having a B.sub.8 value of about 1.92 Tesla have been commercially available. Under the above-mentioned situations, the product of double oriented electrical steel sheet is required to have a magnetization property (B.sub.8 value) corresponding to the above-mentioned grain oriented electrical steel sheet. As processes for improving the magnetic flux density of double oriented electrical steel sheet, a process wherein a hot-rolled material is annealed and then cold-rolled in the mutually rectangular direction is disclosed in Japanese Examined Patent Publication 38-8213, a process wherein a material is nittided in the course from post-primary recrystallization is disclosed to the start of secondary recrystallization is disclosed in Japanese Examined Patent Publication 1-43818, and a process wherein after the cross cold-rolling, the material is further cold-rolled in the initial cold-rolled direction at a reduction rate of 5-33% is disclosed in Japanese Unexamined Patent Publication 1-272718.